Superior Canal Dehiscence

Superior canal dehiscence syndrome (SCDS) is a condition in which vestibular symptoms are elicited by sound or pressure secondary to a dehiscent superior semicircular canal. Imaging studies and vestibular evoked myogenic potential (VEMP) testing are critical in the diagnosis of SCDS. Surgical correction of SCDS is reserved for patients with severe disabling symptoms.

More than 80 years have passed since Tullio and Hennebert described their findings of sound-induced and pressure-induced vestibular activation. ^[1]  Since then, the Tullio phenomenon, wherein vestibular symptoms are induced by loud sounds, has been associated with syphilis, perilymphatic fistula, congenital deafness, Ménière disease, head trauma, Lyme disease, cholesteatomas with labyrinthine fistula, and fenestration operations. The Hennebert sign of vestibular symptoms due to changes in external auditory canal pressure is frequently found in conjunction with the Tullio phenomenon (as in perilymphatic fistula, syphilis, and Ménière disease).

In 2000, Lloyd Minor, MD, then at Johns Hopkins University, found a series of patients with positive Tullio and Hennebert signs. ^[2] He was the first to relate these positive findings directly to an anatomic defect of the superior semicircular canal that was detected with high-resolution computed tomography (HRCT). Minor theorized that, when the bone over the canal becomes thin or dehiscent, it acts as an additional window for the vestibular system, allowing pressure and noise changes to induce vestibular activity. SCDS, as it has become known, is now recognized and treated by otolaryngologists and neurotologists throughout the world.

Symptoms of superior canal dehiscence syndrome

Patients with SCDS usually present with some or all of the symptoms associated with this entity. Most commonly, individuals present with complaints of sound- or pressure-induced dizziness, pulsatile tinnitus, low-frequency hearing loss, hyperacusis, autophonia, or orbital tinnitus. Chronic imbalance or a lightheaded feeling is another symptom of SCDS, but patients usually will have other symptoms associated with this disorder. Symptoms may be recreated when sound or pressure is presented to the affected ear. Patients often control these symptoms with strict avoidance of noisy environments. In addition, some patients may hear a swishing noise when they move their eyes in a certain direction (orbital tinnitus). Moreover, hyperacusis, perceiving a sound louder than it is actually being presented, possibly to the point of pain, although not specific, is found in a lower percentage of patients with SCDS.

Workup in superior canal dehiscence syndrome

Imaging studies and office testing are critical in the diagnosis of SCDS. An HRCT scan of the temporal bones without contrast is required to make a definitive diagnosis; the scan is usually performed with special 1-mm sections through the superior semicircular canal to evaluate the bony covering. 

VEMP testing is another means of confirming a diagnosis of SCDS. This office-based procedure is performed by applying a repetitive click stimulus to an ear and then averaging the reaction of the cervical muscle activity with external electrodes, If the sound in the ear stimulates vestibular-cervical pathways, this can be recorded, indicating a dehiscence of the bony labyrinthine system (SCDS).

Comprehensive audiometric evaluation is indicated in any patient with vestibular symptoms. In a patient with SCDS, key factors may be revealed, one of these being the presence of normal symmetrical hearing, which helps to eliminate a retrocochlear process as a cause of vertigo. Other patients may have low-frequency hearing loss and carry a diagnosis of Ménière disease or atypical Ménière disease. Once the appropriate imaging is performed, the diagnosis may change to SCDS. 

Video-oculography can be valuable in recording the vertical and torsional eye movements specific to SCDS.

Caloric test results are usually unaffected in patients with SCDS; however, when the dehiscence is large (>0.5 mm), reduced caloric test results may be demonstrated on the affected side.

Other tests that may help in the diagnosis of SCDS include vibration of the suboccipital region, with the demonstration of torsional nystagmus.

Management of superior canal dehiscence syndrome

Surgical correction of SCDS is reserved for patients with severe disabling symptoms. Operative techniques include the following:

• Traditional middle fossa craniotomy and repair of fistula
• Endoscopic craniotomy approach - In this procedure, patients undergo a middle cranial fossa craniotomy through a small, limited-access craniotomy of 2 cm or less on the affected side; the temporal lobe is gently retracted; upon elevation of the dura, a small endoscope is gently inserted, and the dehiscence is identified and resurfaced
• Transmastoid superior canal occlusion - In this approach, a mastoidectomy is performed, and the superior semicircular canal is identified near the ossicular heads; the superior semicircular canal is then ablated with a combination of tissue and fascia
• Minimally invasive approach via transcanal oval and round window reinforcement - Rather than directly addressing the dehiscent canal via a middle fossa craniotomy, an innovative approach first described by Kartush suggested that dampening the inner ear's sensitivity by reinforcing the oval and round windows may alleviate symptoms in some patients ^[3]
• Combination approach - In this technique, the superior canal defect is repaired via the middle fossa approach with concomitant reinforcement of the oval and round windows